1. Field of the Invention
This invention relates generally to plant containers and root growth barriers, such as for use with landscape plants such as trees and shrubs prior to transplanting.
2. Background of the Related Art
Transplantable plants for use in landscaping, such as trees and shrubs, are generally initially grown in conventional smooth-walled containers made of plastic, steel or other materials. However, when a root contacts the inner sidewall of a smooth walled container the root is deflected and follows the curvature of the container. The result is the production of only a few secondary branch roots. In some cases, roots make several revolutions around the container, mostly at the bottom, forming a coil. The result is an abnormal root system that does a poor job of establishing and supporting the plant following transplanting. Impaired root development following transplanting restricts growth of any species. Poor root development and anchorage is especially detrimental to trees that grow to considerable height and where stresses from wind, ice and snow are exaggerated.
Whitcomb (U.S. Pat. Nos. 4,442,628; 4,510,712; 4,753,037; 4,939,865; and 5,557,886) teaches various sidewall designs with strategic placement of openings in above ground containers to accomplish air-root-pruning (root tip dehydration pruning) to stop root circling and stimulate root branching. These containers work by guiding an actively growing root tip, which is white soft tissue, into an opening where the root tip dehydrates, dies and is, therefore, effectively pruned. The effect on the root system is the same as when a plant is pruned above ground, for example, to make a hedge. Each time the plant is pruned above ground, secondary branches form back as far as about four inches from the point of pruning and through a succession of prunings, a dense plant hedge or screen can be created. An identical phenomenon is seen to occur with roots for the same physiological reasons. Such air-pruning container designs have been successful and are currently sold in various sizes (available under the Rootmaker™ and RootBuilder™ trademarks from RootMaker Products Co., LLC of Huntsville, Ala.).
Van Wingerden (U.S. Pat. No. 5,131,185), Lawton (U.S. Pat. No. 5,099,607) and Henry (U.S. Pat. No. 5,241,784) also teach air-root-pruning by container sidewall design. Butler (U.S. Pat. No. 5,937,577) teaches air-root-pruning as a result of constructing a container out of chicken wire and lining the inside with a woven polyester fabric. Butler (U.S. Pat. No. 5,937,577) does accomplish air-root-pruning throughout the circumference of the container, but the salt accumulation due to the high loss of water to evaporation makes it less efficient in terms of conserving water. This design is generally only practical to use where irrigation water quality is good and humidity is high.
Whitcomb (U.S. Pat. No. 4,497,132) teaches that when root tips are trapped in a saw tooth-like recess as part of the container wall and cannot extend, root tips cease to grow and root branching results. Root circling is reduced and root branching is improved. However, despite the improvements shown by Whitcomb '132, some spiral root growth is still seen to occur because there are limited number of root-tip-trapping points, and the growth of lateral roots and development of the lateral root tips is not optimal. Once roots occupy all of the root-tip-trapping recesses, as in the '132 container, additional roots may circle and branch poorly in much the same manner as in a conventional container.
Reiger (U.S. Pat. No. 5,768,825) discloses the use of a monolayer of fabric sewn into the shape of a plant preservation and growth control bag. A porous fabric is employed, which restricts the growth of a plant preparatory to transplantation, by catching the roots in the recesses of the fabric. This permits fewer roots to emerge from a layer of fabric, but those that do are prevented from developing further by “girdling” or constriction pruning as a result of fabric entanglement. However, this method is restricted for use during the short time when plants are to be preserved for transplantation and is not suitable for longer-term growth and maintenance. Removal of fabric from plants placed in this system more than a few months is nearly impossible as roots grow through the fabric and develop on the opposite side. Optimal root growth is not maintained because the roots must be broken off at the inside face of the fabric or in the fabric when the fabric is removed. Further, the invention of Reiger ('825) must be used inside a conventional container or in the soil to avoid severe water loss due to evaporation through the fabric.
Therefore, a need exists for a root growth barrier or container for the purposes of encouraging healthy and abundant root growth and permitting optimal development and growth of lateral roots and root tips. It would be desirable if the barrier could provide improved management of water and oxygen within the container. It would also be desirable if the barrier could provide a much greater number of pruning elements, such as root-tip-trapping elements and air-root-pruning elements.